Development of rechargeable high-energy hybrid zinc-iodine aqueous batteries exploiting reversible chlorine-based redox reaction

The chlorine-based redox reaction (ClRR) could be exploited to produce secondary high-energy aqueous batteries. However, efficient and reversible ClRR is challenging, and it is affected by parasitic reactions such as Cl2 gas evolution and electrolyte decomposition. Here, to circumvent these issues, we use iodine as positive electrode active material in a battery system comprising a Zn metal negative electrode and a concentrated (e.g., 30 molal) ZnCl2 aqueous electrolyte solution. During cell discharge, the iodine at the positive electrode interacts with the chloride ions from the electrolyte to enable interhalogen coordinating chemistry and forming ICl3-. In this way, the redox-active halogen atoms allow a reversible three-electrons transfer reaction which, at the lab-scale cell level, translates into an initial specific discharge capacity of 612.5 mAh gI2−1 at 0.5 A gI2−1 and 25 °C (corresponding to a calculated specific energy of 905 Wh kgI2−1). We also report the assembly and testing of a Zn | |Cl-I pouch cell prototype demonstrating a discharge capacity retention of about 74% after 300 cycles at 200 mA and 25 °C (final discharge capacity of about 92 mAh).

Supplementary Note 1. When the capacity calculation involves the anions from the electrolyte, it is a general protocol not to count the ion masses resourcing from the electrolyte.
For a representative example, the cathode capacity to host the AlCl4anions in the aluminum ion battery was calculated based on the mass of the host active materials, such as graphite [13,14] , and the TiSx [15] , without counting the AlCl4ions resourcing from the electrolyte. In addition, regarding the other anions-based batteries, the specific capacity was also calculated based on the masses of the graphite cathode without counting the masses of the anions from the electrolyte [16] .
We have specified that the specific capacity is calculated based on the mass of fixing agents of iodine. The mass of Clwas not counted in the total mass to calculate the specific capacity, because the Clions are resourced from the electrolyte, moving into the cathode side during the charging process and returning to the electrolyte during discharging. The mass of chloride ions involving the electrode reaction is always changing during the charge-discharge process and therefore, the total mass of Cl changes during battery operation, while the mass of I is fixed. Thus, we only used the mass of I, which is regarded as a constant mass value.
Where φ is electrode reaction potential; φ θ is the standard potential; R is the universe gas where the CI is the capacity calculated based on the mass of iodine and CT is the capacity calculated total mass based on iodine and AC. Thus, the corresponding capacity based on the total mass of iodine and AC is 226 mAh g(I+AC) -1 at 0.5 A g -1 , and the corresponding capacities at other specific currents are shown in Supplementary Table 5.
Specific Energy Calculation.. The specific energy was calculated based on the mass of iodine, where the mass of activated carbon (AC) and the mass of Zn anode were not counted in.
Since the mass of electrodeposited iodine is 2.36 mg cm -2 and the mass of AC is 4 mg cm −2 and the mass of zinc foil is 35.9 mg cm −2 (50 μm thickness), the specific energy of the full cell based on the total mass of cathode (mass of iodine and AC) and anode materials would be ET=EI*2.36/(2.36+4+35.9)=0.056EI where the EI is the specific energy calculated based on the mass of iodine and ET is the specific energy calculated based on the total mass. Thus, the specific energy of the Zn||I2 cells based on the positive electrode (mass of iodine and AC) and negative electrode active materials is 50.7 Wh kg -1 .
In order to improve the specific energy of the whole battery, promising strategies can mainly be improved from two aspects, one is to increase the loading of the halogen species or to reduce the mass of hosting materials, and the second is to increase the utilization rate of the Zn anode.

Supplementary Note 7.
The pouch cell was fabricated to demonstrate the scalability of the Cl-I electrode obtained by electrochemical deposition, which could be cycled in the size of 80 cm 2 (10 cm * 8 cm). Other applicable methods to deposit iodine into the host carbon can also be developed, such as thermal-evaporation deposition or immersion deposition method.